1. Field of the Invention
The invention relates to a diagnostic image processing method in which a series of two-dimensional images is formed of an object present in an examination zone, each image of the series being derived from a number of one-dimensional projection images which are formed by projection of the object from one of a plurality of predetermined projection directions and which consist of pixels.
In the MR (magnetic resonance) technique methods of this kind are known in conjunction with the so-called projection-reconstruction method. Such methods are also used in X-ray computer tomography.
2. Description of Related Art
For medical diagnosis it is often necessary to form a series of images of one and the same object, said series representing the object in, for example different phases of motion. The quality of the individual images is then usually limited by noise. In the case of MR methods this noise occurs if the MR signals required for the reconstruction of the images are acquired in an as short as possible period of time. In the case of X-ray CT methods noise occurs whenever only a small dose is used per individual image so as to keep the radiation dose for the patient small.
An article by T.A. Reinen in Proc. SPIE, Vol. 1606, pp. 755-763, 1991, discloses a method which reduces the noise in such a series of images by determining a motion vector field, by applying a noise reduction filter to the pixels (pixel=picture element) linked by the motion vector field, and by taking up the image values resulting from the filtering operation in an output image. This method is based on the consideration that even though the individual images of the series deviate from one another because at least some details are situated in different positions within the images, these details are usually reproduced with the same brightness in the various images. This makes it possible to link two images of the series by way of a motion vector field which describes the shift of these details from one image to another and hence links the content-wise associated pixels (i.e. pixels reproducing the same point of a structure in the various images). The known noise reduction method could also be applied to series of images formed by means of the above-mentioned methods.
It is a drawback of this method, however, that it is comparatively time consuming because for each pixel in an image in prince the content-wise associated pixel within a two-dimensional image window in the other image must be searched. Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.